During the drilling of a well, such as an oil well, progress is monitored by means of periodic measurements and tests. Some are made at the surface; others utilize sophisticated tools which are lowered into the well to make more proximate measurements of well bore parameters. Inferences and deductive evaluations about the well are then made based upon the results of such measurements, made at various depths within the well bore. Obviously, the greater the accuracy of the measurements, the more valid will be the deductions or calculations made from the measurements.
A well-known and important tool for measuring formation pressures and flow rates, and for obtaining one or more fluid samples from the earth formations, is a Formation Tester. When adapted to obtain a number of measurements or fluid samples, it is sometime called a multiple sample formation tester. One such tester, capable of making multiple measurements and taking multiple samples, is disclosed in U.S. Pat. No. 4,375,164 (Dodge et al., issued Mar. 1, 1983), assigned to the assignee of the present invention. As illustrated therein, the tool is adapted to be lowered into a well bore on an armored electrical cable, commonly known as a wire line. At the location in the well bore where a test is desired, a back-up shoe and an elastomeric sealing pad are projected laterally in opposite directions into engagement with opposite sides of the wall of the well bore. The sealing pad seals off a portion of the formation from the well bore, and a channel within the pad, oftentimes including a probe which extends therefrom into the formation, provides direct fluid communication between the tool and the formation interval thereadjacent. The flow channel then effectively opens the formation interval into the tool, where a pressure sensor provides a formation pressure measurement. If desired, a sampling chamber within the tool may also be connected to the formation, as by suitable valves, for obtaining and retaining therein a fluid sample which may then be retrieved at the surface when the tool is withdrawn from the well bore.
Another feature of such tools is the ability to perform pretests before a full fluid sample is drawn. The latter usually amounts to from 0.5 to 10 gallons, and usually can be drawn only once or twice (depending upon the tool configuration) for each trip of the tool into the well. A pretest, however, typically involves drawing only a small fluid sample, usually about 5 to 20 cc. Such samples can be drawn with a piston arrangement in which the fluid can then be purged and the piston used again to draw another sample. Initially, such tests help determine whether a good seal between the pad and the formation has been established. After the integrity of the seal is confirmed, more such pretests can be conducted to provide useful information about the permeability of the formation, as by monitoring the fluid flow rate as a function of the pressure differential generated as the piston draws in the sample.
Such prior art tools and methods, however, conducted as described above, have in fact been conducting pretest measurements, not of the permeability of the formation to its own connate fluids, but of the permeability of the formation to mud filtrate from the bore hole. These can be substantially different values. For example, suppose that the connate formation fluid is a gas. Clearly the gas, which is an inviscid fluid which is compressible, will have markedly different viscosity and flow characteristics from the drilling fluid, which is a somewhat viscous liquid which is incompressible. In such a case the permeability values obtained from a pretest which draws mud filtrate (well bore fluid) can be expected to be very distorted from the actual permeability of the undisturbed formation. This distortion effect can be further enhanced by formation damage in the immediate vicinity of the borehole (where the measurements in fact take place) caused by the well bore drilling fluids and well bore fluid pressures. (This latter change in apparent permeability is known as the "skin effect".)
A need therefore remains for an improved method and apparatus for determining more accurately the flow properties of a formation interval traversed by a bore hole. Preferably, such a method and apparatus will provide permeability information about the formation based upon actual connate formation fluids, and will minimize skin effect and other distortions caused by the well bore fluids.